Motile Aeromonas septicemia (MAS) has been increasingly prevalent in cultured fish throughout China since 1989. Over the past two decades, our laboratory conducted two pathogen detection studies in septicemia outbreak fishponds in an epidemic area in central China. One was conducted from May 1990 to October 1991, when MAS was beginning to spread in China, and the other was recently conducted from August 2006 to July 2009. We found that Aeromonas hydrophila was responsible for these MAS outbreaks. A. hydrophila isolates were previously known to be phenotypically, serologically, and genetically diverse and no dominant clones were found. In this study, multilocus sequence typing analysis was used to observe a clonal lineage of A. hydrophila, which was responsible for MAS outbreaks in pond-cultured cyprinid fish in an epidemic area in central China for two decades. (C) 2014 Elsevier B.V. All rights reserved.